Contains reports on four research projects

Campbell, J. F., Jr.

Greenburg, J.

Nottingham, W. B.

Stettler, J. D.

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I. PHYSICAL ELECTRONICSProf. W. B. Nottingham J. F. Campbell, Jr. C. D. RobbinsE. Ahilea D. S. Dunavan L. E. SpragueD. A. Berkowitz J. Greenburg J. D. StettlerA. PHYSICAL ELECTRONICS IN THE SOLID STATE1. Characteristics of Junctions in GermaniumMeasurement of diode characteristics at very low voltages provides a method forevaluating that part of the diode resistance that is independent of the applied voltage.In the region of interest the current i through the junction itself may be related to thevoltage V across the junction byi (e= V/VT 1(1)where VT = kT/e. The quantity i0 is related to the ideal reverse saturation current ofthe diode and may be determined from the measured voltage-current characteristic.For V << V T , Eq. 1 reduces toii 0 V = o V (2)V o0and the diode becomes an ohmic conductor with a conductance a- . But if the diode has0a resistance R effectively in series with it as a result of the resistance of the basematerial, then Eq. 2 becomesi= a- (V - iR) (3)0 0V being the voltage that is actually measured. Thus, the effect of R is to shift themeasured characteristic along the voltage axis by an amount iR. The quantity 0o isindependent of R, however, and can still be determined. If we let o = i/V o , thenEq. 3 giveso0from which we have1 1R-This method has been applied to previously determined diode characteristics, andit yields reasonable values having approximately the correct temperature dependence.With the use of a recently constructed potentiometer, stray voltages from humpickup have been almost entirely eliminated, and measurement of the voltage on a(I. PHYSICAL ELECTRONICS)sample of 105 ohms can be made to an accuracy of 1 pvolt or better. A new tempera-ture regulator that uses a chopper amplifier to control the heater voltage of the sampleoven and a thermocouple as the sensing element has been constructed. Performancetests indicate that this regulator will hold the sample temperature constant within0.0250 C for periods as long as an hour.J. F. Campbell, Jr.B. ELECTRON EMISSION PROBLEMS1. Studies of a Planar Dispenser CathodeA few samples of a dispenser cathode of the capillary type have been furnished bySiemens and Halske for study. The emitting surfaces are planar and have a diameter of3 mm. Two of these cathodes were mounted "face to face" with a separation of 1 mm.Either one could be used as the emitter and the other as the collector. A detailedstudy of the current-voltage relation interpreted by the theories and correspondingtables given in Technical Report No. 321, Research Laboratory of Electronics, M.I.T.,(reprinted from the "Thermionic Emission" section of Vol. 21, Handbuch der Physik,1956), permits the determination of the emitter temperature as a function of heaterpower. The receiver work-function can be measured to within one hundredth of anelectron volt. This method of analysis also permits the study of variations in thework-function of the cathode when it is used as a receiver to show changes that occuras a result of deposits that come from the emitter structure. The receiver can alsobe operated at relatively high temperatures and in that manner determine variationsin the work-function with temperature that depend not only on the changing Fermilevel but also on the changing physical state of the collector.W. B. Nottingham2. Effect of a Magnetic Field and of a Unidirectional Heating Currenton Thermionic Emission from MolybdenumRecent measurements on the magnetic field effect show a decrease in total plate cur-rent with increasing applied field. For a field of 6000 gauss this decrease is 13 per centfor the current to the four anodes, and 9 per cent in the case of the two guard rings.The current remains constant up to fields of about 400 gauss; thereafter the decrease isroughly proportional to the increase in field.We must now ascertain whether this decrease represents a drop in the emissioncurrent itself or a change in collector current caused by modification by the field of thetrajectories of emitted electrons. With the present tube structure, the latter possibil-ity cannot be entirely ruled out; the presence of slits between the plates and guard ringsaffords a possible means of escape for these electrons.(I. PHYSICAL ELECTRONICS)For this reason, a new, smaller tube has been designed. It preserves the originalplan of four separate plates surrounding the filament, but eliminates all apertures andother irregularities along the length of the channel. The new design consists of fourflat plates that extend the full length of the filament and overlap in such a way as toinsure collection of the total emission current. With this structure, any decrease inplate current could not be attributed to changes in collection efficiency and could there-fore be interpreted as a true variation in emission current.J. GreenburgC. EXPERIMENTAL TECHNIQUES-121. Electrometer for Currents down to 10 ampereThe frequency response of the Shelton electrometer has been extended by mechani--10cal redesign of the input circuit to approximately 300 cps on the 10 -amp scale. Thefeedback part of the amplifier has also been improved to eliminate an "overshoot" andoscillation found in one of the earlier designs.J. D. Stettler2. Use of an Internal Disappearing-Filament Pyrometer inTemperature MeasurementsSince the readings of a conventional pyrometer are dependent on the transparencyof the tube walls, and since this transparency may be drastically reduced by evapora-tion of metal or oxides, an optical means of temperature determination which is inde-pendent of the transmission of the glass would be useful. In this investigation, anadditional filament was mounted within the envelope, placed so that it could be viewedsuperimposed on the emitting filament and therefore used as an internal disappearing-filament pyrometer.The pyrometer filament was a straight tantalum wire, 4. 5 cm long and 0. 250 mmin diameter. For such a filament, the temperature can be computed fairly accuratelyin terms of the heating current (1). The emitter was a flat ribbon of molybdenum. Intaking a reading, the two filaments were viewed simultaneously through a monochro-matic filter, and the pyrometer filament current was varied until the two were of equalbrightness. The current was then recorded, and the true temperature calculated fromthe tables in reference 1. The brightness temperature of the pyrometer filament (andhence of the emitting filament) can then be computed. A final calculation yields thetrue temperature of the emitter. The equation for conversion from true to brightnesstemperature for the pyrometer filament, and from brightness to true temperature forthe emitter, is (2)(I. PHYSICAL ELECTRONICS)1 1 X loge ekTT T C 2o B where X is the filter wavelength, eXT the spectral emissivity at that wavelength andtemperature, and C2 = 1.438 cm-°K. In practice these conversions were performedvery simply by graphical means.The results thus obtained were compared with the readings of a Leeds and Northrupoptical pyrometer, corrected for the transmission of the glass. At temperaturesabove 18000 K, the internal pyrometer read somewhat lower than the external, theaverage deviation being 90 K. At lower temperatures, the internal readings becameprogressively higher. This is probably due to the effect of end cooling of the pyrom-eter filament, which produces an erroneously high value of temperature. The usefulrange of the method could be extended by making a correction for this end cooling; thisrequires measurement of the filament voltage drop (3).The deviation in the upper range may well be due to uncertainty in the spectralemissivity of molybdenum, which according to the source used may cause deviationsof 10 to 150 (2). In cases where spectral emissivity is more accurately known, I feelthat this method, with a few further refinements, could be used to good advantage.J. GreenburgReferencesi. L. Malter and D. B. Langmuir, Resistance, emissivity, and melting point oftantalum, Phys. Rev. 55, 743 (1939).2. G. J. Baldwin, J. L. Shilts, and E. A. Coomes, Temperature scale for molybdenum,Technical Report on Contract Nonr 1623(00)372-731, Notre Dame Physical Elec-tronics Research Reprint Series, June 1955.3. I. Langmuir, S. MacLane, and Katharine B. Blodgett, The effect of end losses onthe characteristics of filaments of tungsten and other materials, Phys. Rev. 35,478 (1930).

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